1. Field of the Invention
The present invention relates to a composite material for anode which enables production of a lithium secondary battery large in charge-discharge capacity, superior in safety and excellent in charge-discharge cycle property; a process for production of the composite material for anode; an anode produced by using the composite material for anode; and a lithium secondary battery produced by using the the composite material for anode.
2. Description of the Related Art
As electronic appliances have become smaller and lighter, the batteries used therein are required to have a higher energy density. The batteries used therein are also required to allow repeated charge and discharge, from the standpoint of resource saving. In order to respond to these requirements, secondary batteries using lithium were proposed and developed.
Secondary batteries using lithium used metallic lithium as the anode material, at the initial stage of development of such batteries. Secondary batteries using metallic lithium as the anode, i.e. metallic lithium secondary batteries, however, have problems of being inferior in charge speed and short in cycle life. These metallic lithium secondary batteries further have a problem in safety because they generate dendrite and the dendrite may cause combustion and explosion. Hence, currently, lithium secondary batteries using a carbon-based material and/or a graphite-based material as the anode, i.e. lithium ion secondary batteries are in wide practical use.
In order to allow the lithium ion secondary batteries and other lithium secondary batteries to have higher discharge capacities, researches are being continued on the cathode material, anode material and electrolyte used therein. As the cathode material, LiCoO2 has been used widely. It is because LiCoO2 is easy to produce, has high stability at high temperatures, and possesses relatively high safety.
Recently, it has been investigated to produce a cathode using LiNiO2 having a larger theoretical discharge capacity than LiCoO2 has.
Regarding the anode material, metallic lithium has a theoretical discharge capacity of 4,000 mAh/g which is far larger than that (372 mAh/g) of graphite. Hence, vigorous researches are under way on lithium secondary batteries using metallic lithium as the anode material, in order to obtain a lithium secondary battery of high discharge capacity free from any problem in cycle life or safety. A research is also under way on the use of a lithium alloy having a discharge capacity close to that of metallic lithium, as an anode material.
Also, various studies on the electrolyte are under way. These studies include the improvement in solid electrolyte of lithium solid secondary battery and the improvement in polymer electrolyte of polymer lithium secondary battery.
It is no exaggeration to say that the improvement in discharge capacity of lithium secondary battery depends on the improvement in discharge capacity of anode material used in the battery.
As mentioned above, it is being attempted in the research of lithium secondary battery to use a lithium alloy as the anode material of battery. As the lithium alloy, there can be mentioned, for example, a lithium-tin alloy, a lithium-lead alloy, a lithium-bismuth alloy, a lithium-aluminum alloy, a lithium-arsenic alloy, a lithium-silicon alloy and a lithium-antimony alloy.
One of the above alloys may be used per se as an anode material to produce a lithium battery. In many cases, however, a metal or semimetal capable of forming an alloy with lithium is used as an anode material, to produce a battery. During the charging of the battery produced, the metal or semimetal is allowed to react electrochemically with the lithium released from the cathode, in the battery and becomes an alloy, whereby an anode material made of a lithium alloy is formed.
In this method, however, the volume of the anode material expands, during alloying, to several times the volume of the anode material before alloying, which causes powdering of the anode material. As a result, there is no sufficient improvement in safety and cycle property of battery. Therefore, no lithium secondary battery using a lithium alloy as the anode material is in practical use currently.
The biggest problem appearing when a lithium alloy is used as the anode material of battery, is that, as described above, the anode volume expands at the time of lithium alloy formation, causes powdering and resultant anode destruction.
The present inventors made a study and found out that covering of a metal or semimetal capable of forming a lithium alloy, with carbon can prevent the powdering of anode material and the consequent destruction of anode.
That is, by using particles of a metal or semimetal capable of forming a lithium alloy, as nuclei and covering each of the nuclei with carbon, there can be obtained a composite material having a double structure consisting of particle nucleus and a covering layer.
When a lithium secondary battery is formed using this composite material as the anode material and is charged, the particle nuclei in the anode material, i.e. the metal or semimetal changes into a lithium alloy. In the change, however, the expansion of the anode material is suppressed by the large restrictive force of the carbon covering layer formed on the nuclei, and the powdering of anode material and the destruction of anode are prevented.
The covering of particle nuclei with carbon can be conducted by various methods. Each method, however, must be such that the covering layer formed has a strength capable of sufficiently suppressing the volume expansion of particle nuclei associated with the alloying of particle nuclei and moreover can cover each of the particle nuclei uniformly and completely.
As a result of a study, the present inventors found out that, of the various covering methods employable, a method of covering particle nuclei with carbon by chemical vapor deposition can generate a restrictive force capable of sufficiently suppressing the volume expansion of particle nuclei and moreover can cover the particle nuclei uniformly and completely with a small amount of carbon. Therefore, this method was found to be a particularly preferred method for covering particle nuclei.
It was found out that preferred as the material for particle nuclei are titanium, iron, boron, silicon, etc. selected from metals and semimetals each capable of forming a lithium alloy and particularly preferred is silicon (JP-A-2000-215887).
When silicon (as particle nuclei) was covered with carbon to form a composite material and a battery was produced using the composite material as an anode material and subjected to charge and discharge, however, it was found that no high charge-discharge speeds are obtainable because silicon has low electrical conductivity and its reaction with lithium is non-uniform.
It was also found that although a carbon covering layer formed on silicon (particle nuclei) has a large restrictive force capable of suppressing the expansion of particle nuclei associated with the alloying, the restrictive force becomes insufficient in repeated charge and discharge when the carbon amount of the carbon covering layer is small.
Hence, it was proposed to use, as particle nuclei, a mixture of silicon and a highly conductive addition element, in place of silicon alone (Japanese Patent Application No. 2000-92810).
By the action of the addition element, the intercalation of lithium into particle nuclei and alloying of silicon with lithium became uniform, making it possible to obtain higher charge and discharge speeds.
The action of the addition element also restricted the expansion of particle nuclei associated with alloying. As a result, the expansion of particle nuclei was restricted by the two actions of the addition element and the carbon of the covering layer, whereby the powdering of anode material and the destruction of anode could be prevented more reliably.
Even in a secondary battery using such an anode material as the anode, however, it was found that the voltage at the completion of charge operation is unstable, rapid heat generation, dendrite formation, etc. occur depending upon the battery use conditions, and the secondary battery may have a problem in safety.
In view of the above situation, the present invention aims at solving the problems of the prior art and providing a composite material for anode of lithium secondary battery, large in discharge capacity, superior in safety and excellent in charge-discharge cycle property, a process for producing the composite material, an anode using the composite material, and a lithium secondary battery using the composite material.
The above aims are achieved by the following inventions.
[1] A composite material for anode of lithium secondary battery, comprising:
a porous particle nucleus formed by bonding of at least silicon-containing particles and carbon-containing particles, and
a carbon-made covering layer formed thereon.
[2] A composite material for anode of lithium secondary battery, according to [1], having an average particle diameter of 0.1 to 50 xcexcm and a specific surface area of 5 m2/g or less.
[3] A composite material for anode of lithium secondary battery, according to [1], wherein the proportion of the covering layer in the composite material is 5 to 60% by mass.
[4] A composite material for anode of lithium secondary battery, according to [1], wherein the silicon content in the porous particle nucleus is 10 to 90% by mass.
[5] A composite material for anode of lithium secondary battery, according to [1], wherein the carbon-containing particles have a specific resistance of 1.0 xcexa9xc2x7cm or less.
[6] An anode of lithium secondary battery, obtained by adhering a composite material for anode of lithium secondary battery set forth in any of [1] to [5], to a current collector.
[7] A lithium secondary battery containing, in the anode, a composite material for anode of lithium secondary battery set forth in any of [1] to [5].
[8] A process for producing a composite material for anode of lithium secondary battery, which comprises:
a particle nuclei-producing step of subjecting a mixture of silicon and carbon to grinding and granulation to produce porous particle nuclei, and
a covering layer-forming step of forming a covering layer on each of the porous particle nuclei produced above.
[9] A process for producing a composite material for anode of lithium secondary battery, according to [8], wherein the carbon has a specific resistance of 1.0 xcexa9xc2x7cm or less.
[10] A process for producing a composite material for anode of lithium secondary battery, according to [8] or [9], wherein the covering layer-forming step is conducted by chemical vapor deposition.
The composite material of the present invention comprises a porous particle nucleus in which silicon-containing particles and carbon-containing particles are finely dispersed in each other and bonded to each other. In this porous particle nucleus, both silicon-containing particles and carbon-containing particles are dispersed in fine particles. Therefore, the porous particle nucleus has high conductivity, enabling uniform intercalation of lithium ion into silicon and uniform alloying. As a result, a lithium secondary battery produced by using the composite material containing such a porous particle nucleus can have high charge and discharge speeds.
The porous particle nucleus has appropriate voids inside, and the voids absorb the volume expansion taking place during alloying of silicon. This can reduce the amount of carbon required for suppression of the volume expansion taking place during alloying of silicon. As a result, the amount of carbon in the covering layer of the composite material can be reduced, resulting in larger charge and discharge capacities per mass.
When a lithium secondary battery is produced using such a composite material and charged, the final voltage of the battery is stabilized at about the same level as that of the carbon particles surrounding silicon particles; therefore, in the battery, generation of dendrite can be prevented. Thus, by using such a composite material as an anode material of lithium secondary battery, there can be provided a lithium secondary battery having high safety, a large discharge capacity and excellent charge-discharge cycle property.